A magnetic disk drive includes circular data tracks on a rotating magnetic disk and read and write heads that may form a merged head on a slider that is attached to a positioning arm. During a read or write operation, the merged head is suspended over the magnetic disk on an air bearing surface (ABS). The sensor in a read head is a critical component in which different magnetic states are detected by passing a sense current there through and monitoring a resistance change. One form of magneto-resistance is a spin valve magnetoresistance (SVMR) or giant magnetoresistance (GMR) which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. In the absence of an external magnetic field, the magnetization direction of the free layer is aligned perpendicular to that of the pinned layer by the influence of abutting hard bias layers. When an external magnetic field is applied by passing the sensor over a recording medium on the ABS plane, the magnetic moment of the free layer may rotate to a direction which is parallel to that of the pinned layer. A sense current is used to detect a resistance value which is lower when the magnetic moments of the free layer and pinned layer are parallel.
In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the sensor stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the cross-sectional area of the sensor is typically smaller than 0.1×0.1 microns at the ABS plane. Current recording head applications are typically based on an abutting junction configuration in which a hard bias layer is formed adjacent to each side of a free layer in a GMR spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important so that edge demagnification in the free layer is prevented. In other words, horizontal (longitudinal) biasing is necessary so that a single domain magnetization state in the free layer will be stable against all reasonable perturbations. The critical dimensions for sensor elements become smaller with higher recording density requirements and therefore the minimum longitudinal bias field necessary for free layer domain stabilization increases.
A high coercivity in the in-plane direction is needed in the hard bias layer to provide a stable longitudinal bias that maintains a single domain state in the free layer and thereby avoids undesirable Barkhausen noise. By arranging the flux flow of the free layer to be equal to the flux flow of the hard bias film, there are no magnetic poles at the abutting junction edges and the demagnetizing field in that vicinity becomes zero. This condition is realized when there is a sufficient in-plane remnant magnetization (Mr) which may also be expressed as Mrt since Mr is dependent on the thickness of the hard bias layer. Mrt is the component that provides the longitudinal bias flux to the free layer and must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of a reasonably sized external magnetic field. Moreover, a high saturation magnetization (Ms) and a high squareness (S) value for Mr/Ms that approaches 1 in the hard bias layer is desired.
Referring to FIG. 1, a conventional read head 1 based on a GMR configuration is shown and is comprised of a substrate 2 upon which a first shield layer 3 and a first gap layer 4 are formed. There is a GMR element comprised of a bottom portion 5a, a free layer 6, and a top portion 5b formed on the first gap layer 4. Note that the GMR element generally has sloped sidewalls wherein the top portion 5b is not as wide as the bottom portion 5a. The GMR element may be a bottom spin valve in which an AFM pinning layer and pinned layer (not shown) are in the bottom portion 5a or the GMR element may be a top spin valve where the AFM and pinned layers are in the top portion 5b. There is a seed layer 7 formed on the first gap layer 4 and along the GMR element which ensures that the subsequently deposited hard bias layers 8 have a proper microstructure. Hard bias layers 8 form an abutting junction 12 on either side of the free layer 6. Leads 9 are provided on the hard bias layers 8 to carry current to and from the GMR element. The distance between the leads 9 defines the track width TW of the read head 1. Above the leads 9 and GMR element are successively formed a second gap layer 10 and a second shield layer 11.
The pinned layer in the GMR element is pinned in the Y direction by exchange coupling with an adjacent AFM layer that is magnetized in the Y direction by an annealing process. The hard bias layers 8 which are made of a material such as CoCrPt are magnetized in the X direction as depicted by vectors 13 and influence an X directional alignment of the magnetic vector 14 in the free layer 6. When a magnetic field of sufficient strength is applied in the Y direction from a recording medium by moving the read head 1 over a hard disk (not shown) in the Z direction, then the magnetization in the free layer switches to the Y direction. This change in magnetic state is sensed by a voltage change due to a drop in the electrical resistance for an electrical current that is passed through the MR element. In a CIP spin valve, this sense current Is is in a direction parallel to the planes of the sensor stack.
One concern about the output from a spin valve element during a feed back (read) operation is that the asymmetry sigma should be as small as possible in order to accurately reproduce the waveform from the recording medium. Asymmetry is determined by the variable magnetization direction of the free layer. Ideally, the magnetic moment 14 of the free layer 6 is orthogonal to the magnetic moment of the pinned layer when no external magnetic field is present. However, the actual angle between the aforementioned magnetic moments usually deviates somewhat from 90° because of other magnetic forces in the GMR element and thereby produces an asymmetric waveform in the output.
A soft magnetic film with a high saturation flux density and comprised of FeCoMo is employed as a magnetic pole layer in U.S. Patent Publication 2002/0150790. Referring to FIG. 2, those skilled in the art would recognize that a FeCoMo layer can be used to modify the read head in FIG. 1 by inserting a FeCoMo underlayer 15 with a high magnetic moment between the seed layer 7 and the hard bias layer 8 (FIG. 2). The underlayer 15 improves the biasing efficiency and serves to reduce output asymmetry. A further improvement in signal amplitude and asymmetry is expected if the FeCoMo layer (or moment) can be increased while the total Mrt is maintained or further reduced and the coercivity is maintained. Unfortunately, the thickness of a FeCoMo underlayer and the associated magnetic moment contribution is limited because a thicker FeCoMo film leads to a loss of coercivity (Hc) in the hard bias layer 8. Thus, a new hard bias structure is needed which allows the thickness of a body centered cubic (BCC) underlayer such as FeCoMo to be increased without lowering Hc in the adjacent hard bias layer. A combination of high coercivity to enhance edge junction pinning efficiency and a higher moment contribution from a BCC underlayer to further reduce asymmetry sigma has not been achieved in prior art hard bias structures to our knowledge.
In U.S. Pat. No. 6,643,107, the electrode layers on both sides of a GMR element are extended toward the center of a bottom spin valve and are located above a backed (conductive) layer on a free layer. This structure prevents dead zones at the edges of the free layer and improves output characteristics including asymmetry.
An MR sensor is described in U.S. Pat. No. 6,270,588 in which the Hex angle that is the angle between the direction of the exchange coupling magnetic bias applied to the pinned layer and the longitudinal bias direction is more than 90° in at least a portion of the pinned layer. As a result, improved wave shape and better wave symmetry is achieved.
An insulating hard bias layer made of cobalt ferrite or the like is used in U.S. Pat. No. 6,512,661 to avoid shunting of current away from a MR sensor which occurs with a conductive hard bias layer. A larger flux decay length is also provided which leads to a higher density recording capability.
In U.S. Pat. No. 6,519,121, a spin valve sensor with a composite pinned layer to improve biasing of the free layer is described. A CoFeHfO layer is formed on an AFM layer and a CoFe layer is formed on the CoFeHfO layer and adjacent to a spacer layer in a MR element. This configuration minimizes sense current shunting and improves the magnetoresistive effect.